1. Field of the Invention
This invention relates to an optical module used to transmit and receive optical signals in optical communications systems.
2. Description of Related Art
In general, optical signals which have been transmitted through optical fibers are received by the input ports of receivers, and converted into electric signals by photo detector modules using, for example, photo diodes (PD).
The size of the detection area of the photo detector used in such a photo detector module is very small (having a diameter of about 80 xcexcm. In order to efficiently receive the optical beam emitted from the optical fiber into the photo detecter, a collecting lens is typically used. For example, a technique of aattaching a ball lens directly to the cap of the photo detecter is known as a low-cost light-collecting method.
FIG. 1 illustrates an example of this type of photo detecter module. This photo detecter module comprises a commercially available stem 2, which is generally referred to as a TO-46 type stem, and a photo detecter 1 mounted on the stem 2. The stem 2 is fixed to a cap 3 so that the photo detecter 1 is accommodated in the cap 3. A ball lens 4 made of low-temperature-melting glass is attached to the middle of the cap. The cap 3 is fixed to a holder 5 by a bond 6, such as adhesive or solder.
An optical fiber 7 is connected to the holder 5 via a sleeve 9. To be more precise, the optical fiber 7 is inserted in a ferrule 8, and the ferrule 8 is inserted in and fixed at the sleeve 9 by YAG welding. As a result of the YAG welding, nuggets 10 are formed at the boundaries between the holder 5 and the ferrule 8, and at the boundaries between the ferrule 8 and the sleeve 9.
In assembling the conventional photo detector, first, the photo detector 1 is bonded to the stem 2. Then, the stem 2 is fitted into the cap 3, to which the ball lens 4 has already been fixed, and the stem 2 is fixed to the cap 3 by resistence-welding.
Then, the holder 5 is fixed to the cap 3 at the end opposite the stem 2 by a bond 6. The cap 3 can not be directly YAG welded because the cap 3 is press-processed.
Finally, the optical fiber 7 is positioned in the optimal position by an optical alignment technique, and then fixed to the holder 5, via the sleeve 9, by YAG welding.
When this photo detector module is used for high-speed transmission in a STM system, the optical signal which has been propagated through the optical fiber 7 must be prevented from being reflected by the surface of the photo detector 1 back to the transmission path. For this reason, the end surfaces of the ferrule 8 and the optical fiber 7, which face the surface of the ball lens 4, are slanted by grinding or polishing for the purpose of inclining the incident angle of the beam onto the photo detector 1, and for the purpose of reducing the return light to the optical fiber 7.
In particular, if the inclination of the end surface of the optical fiber 7 is set to about 12 degrees, the incident angle of the beam on the photo detector 1 becomes about 6 degrees, which can reduce the reflected component of the signal light to less than xe2x88x9240 dB.
However, this arrangement causes a large variation in the quantity of the reflected light because the coaxiality between the photo detector 1 and the ball lens 4 is lost.
FIGS. 2(1) through 2(3) show the optical coupling and the coaxiality between the photo detector 1 and the ball lens 4.
FIG. 2(1) illustrates the optical connection at the optimal position with little separation between the ball lens 4 and the photo detector 1. In this arrangement, the light emitted from the optical fiber 7 strikes the photo detector 1 at an optimal incident angle, and the amount of the light reflected back to the optical fiber 7 via the ball lens 4 is greatly reduced. In this arrangement, the orientation of the end surface of the optical fiber 7 does not affect the physical properties of the optical connection between the ball lens 4 and the photo detector 1.
However, if the optical axes of the photo detector 1 and the ball lens 4 are offset from each other, as shown in FIGS. 2(2) and 2(3), the quantity of reflected light increases, while the coupling efficiency decreases.
For example, if the separation between the photo detector 1 and the ball lens 4 and the orientation of the slanted end surface of the optical fiber 7 satisfy particular conditions (that is, if the orientation of the end surface of the optical fiber 7 is 12 degrees, and if the separation is about 0.11 mm, as shown in FIG. 2(2)), then a problem arises. In the example of FIG. 2(2), if the optical fiber 7 is aligned at the optimal position, the light which exits obliquely from the optical fiber 7 strikes the surface of the photo detector 1 at a normal angle. The normal incident light is reflected by the photo detector, and returns along the same path as the incident light to the optical fiber 7. The amount of reflected light may reach a maximum in this arrangement.
On the other hand, as shown in FIG. 2(3), if the orientation of the inclined end surface of the optical fiber 7 is opposite the orientation shown in FIG. 2(2), the quantity of reflected light is reduced. However, in this arrangement, the light emitted from the optical fiber 7 passes through the ball lens 4 at a position offset from the center axis of the ball lens 4. As a result, the spherical aberration of the ball lens 4 adversely affects the system, which reduces the optical coupling efficiency and causes the light-collecting ability of the photo detector to deteriorate.
In addition to these problems, the coaxiability between the photo detector 1 and the ball lens 4 of the conventional photo detector module may be offset by 0.25 mm at most due to the following factors:
(1) Because there is no target or alignment mark on the stem 2, the photo detector 1 can not be precisely bonded;
(2) Because the cap 3 is press-processed, its dimensional stability is insufficient and, accordingly, a large clearance is required at the portion for receiving the stem 2, which causes the optical axes of the stem 2 and the cap 3 to be offset from each other during the resistance welding step; and
(3) It is mechanically difficult to accurately fix the ball lens 4 in the center of the cap 3.
If the separation between the photo detector 1 and the ball lens 4 is large, the amount of reflected light and the coupling efficiency vary greatly with the slanting orientation of the end surface of the optical fiber 7. In order to achieve a predetermined desired performance, employing an optical alignment technique for setting the orientation of the slanting end surface of the optical fiber 7 at the optimal angle, while monitoring the reflected light and the coupling efficiency, is indispensable. However, such an optical alignment technique requires a number of steps, and the production yield is lowered as a result
In addition, it is also difficult in the conventional photo detector module to precisely position the holder 5 with respect to the cap 3 along the optical axis (i.e., the z-axis) because of the existence of adhesive or solder. Since the optical fiber 7 must be precisely positioned both in the direction perpendicular to the optical axis and in the direction parallel to the optical axis, the ferrule 8 and the holder 5 need to be secured at two positions via the sleeve 9 by YAG welding.
Furthermore, an adhesive or solder is also used to fix the holder 5 to the cap 3, which is not preferable for maintaining the reliability of the optical module for a long period of time.
Therefore, it is an object of the invention to overcome these problems in the prior art, and to provide an optical module which comprises a stem having an interconnection terminal; an optical device positioned on the stem and electrically connected to the interconnection terminal; a cap fixed onto the stem so as to enclose the optical device within the cap; and a lens for optically coupling the optical device with an optical member positioned outside the cap. As a feature of this optical module, the cap is manufactured with a high degree of precision so that the height of the cap with respect to the stem can be used as a positional reference in the direction of the optical axis of the optical system, which consists of the optical device, the lens, and the optical member extending outside the cap.
In addition, the shape of the side wall of the cap is designed so that it can function as a reference for maintaining the coaxiality between the photo detector and the lens. The stem is fixed to the cap at the proper position for a high precision.